Publisher Summary Thermal control of electronic components has one principal objective, to maintain relatively constant component temperature equal to or below the manufacturer's maximum specified service temperature, typically between 85 and 100°C. It is noted that even a single component operating 10°C beyond this temperature can reduce the reliability of certain systems by as much as 50%. Therefore, it is important for the new thermal control schemes to be capable of eliminating hot spots within the electronic devices, removing heat from these devices and dissipating this heat to the surrounding environment. Several strategies have developed over the years for controlling and removing the heat generated in multichip modules, which include advanced air-cooling schemes, direct cooling, and miniature thermosyphons or free-falling liquid films. The chapter summarizes analytical, numerical, and experimental work in literature, in order to facilitate the improvement of existing schemes and provide a basis for the development of new ones. The chapter focuses on investigations performed over the past decade and includes information on the thermal control of semiconductor devices, modules, and total systems.

Thermal Control of Electronic Equipment and Devices

Optimization of Finned Heat Sinks for Impingement Cooling of Electronic Packages

Free stream cooling of a stack of parallel plates

It was thought that the invention of the transistor, with its relatively low power requirements, would greatly minimize, if not totally eliminate, all cooling concerns. Such thoughts, however, were short lived, as engineers sought to improve performance, cost, and reliability by packaging greater numbers of circuits in an ever-smaller space. In fact, power densities at the component level have increased dramatically over the years. In mainframe computers, chips may be found with power dissipations ranging between 20 and 40 W, and chips with power dissipation in excess of 10 W may be found in many PC and workstation applications. Considering one example from a mainframe computer, a 7 × 7-mm chip dissipating 30 W, results in a heat flux of more than 6 × 105 W/m2. As shown in Figure 4-1, this is only about two orders of magnitude less than that on the surface of the sun [1]. But the sun’s surface temperature is 6000°C, compared to a maximum operating temperature in the range of 100°C for a typical semiconductor chip.

Heat Transfer in Electronic Packages

Experiments have been performed using water and FC-77 to investigate heat transfer from an in-line 1 x 10 array of discrete heat sources, flush mounted to protruding substrates located on the bottom wall of a horizontal flow channel. The data encompass flow regimes ranging from mixed convection to laminar and turbulent forced convection. Buoyancy-induced secondary flows enhanced heat transfer at downstream heater locations and provided heat transfer coefficients comparable to upstream values. Upstream heating extended enhancement on the downstream heaters to larger Reynolds numbers. Higher Prandtl number fluids also extended heat transfer enhancement to larger Reynolds numbers, while a reduction in channel height suppressed buoyancy driven flows, thereby reducing enhancement. The protrusions enhanced the transition to turbulent forced convection, causing the critical Reynolds number to decrease with increasing row number. The transition region was characterized by large heater-to-heater variations in the average Nusselt number.

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Liquid Immersion Cooling of a Longitudinal Array of Discrete Heat Sources in Protruding Substrates: I—Single-Phase Convection

The resistance to heat flow from finned LSI packages to the cooling air flow has been studied by combining the physical models with the results of heat transfer experiments, flow visualization experiments, and pressure drop measurements. Although crude assumptions are employed in analytical modeling, the proposed method of prediction of heat transfer and pressure drop has proved useful to cooling system designers. A notable finding is the advantage obtained by placing the packages in a staggered arrangement on the card. This reduces the local air temperature rise by as much as 70% over the conventional in-line arrangement, where the fan power is given as a constraint.

N. Ashiwake

Papers

Forced convective heat transfer from LSI packages in an air-cooled wiring card array